Ammonium-nitrogen contained in wastewater is one of causative agents of eutrophication in such as a river, a lake, and sea and should be effectively removed by wastewater treatment process. Ammonium-nitrogen in wastewater is decomposed into nitrogen gas through two-stage biological reaction processes. In a first nitrification process, the ammonium-nitrogen is oxidized into nitrite-nitrogen by ammonia oxidizing bacteria, and the nitrite-nitrogen is further oxidized into nitrate-nitrogen by nitrite oxidizing bacteria. In a second denitrification process, the nitrite-nitrogen and the nitrate-nitrogen are decomposed into nitrogen gas by denitrifying bacteria as heterotrophic bacteria where an organic substance as an electron donor is consumed.
In the conventional nitrification-denitrification process, cost of aeration for oxidizing the ammonium-nitrogen makes up the majority of the operational cost.
In the conventional nitrification-denitrification method, a large amount of an organic substance such as methanol as the electron donor is consumed in the denitrification process which produces a large amount of sludge.
A method where ammonium-nitrogen is oxidized to nitrite-nitrogen without producing nitrate-nitrogen and then the nitrite-nitrogen is denitrified may reduce its cost.
Another denitrification method is known in which ammonium-nitrogen and nitrite-nitrogen are reacted with each other by autotrophic microorganisms (hereinafter, sometimes referred to as “ANAMMOX bacteria”), wherein the ammonium-nitrogen is an electron donor and the nitrite-nitrogen is an electron acceptor. According to this method, no organic substance is added, thereby the cost could be reduced comparing to the conventional method utilizing heterotrophic denitrifying bacteria. The autotrophic microorganisms generate lower amount of sludge than that of the heterotrophic microorganisms because of lower yield of the autotrophic microorganisms, thereby reducing the amount of excess sludge compared to the method utilizing heterotrophic denitrifying bacteria. No N2O is produced during this process in contrast to the conventional nitrification-denitrification process, thereby reducing influence to the environment.
The biological denitrification process utilizing ANAMMOX bacteria was reported in Strous, M, et al., Appl. Microbiol. Biotechnol., 50, p. 589-596 (1998). The ammonium-nitrogen and nitrite-nitrogen are considered to be decomposed into nitrogen gas in the following reaction where ammonium-nitrogen and nitrite-nitrogen react with each other in a ratio of ammonium-nitrogen/nitrite-nitrogen=0.43/0.57 (molar ratio)=1/1.3 (molar ratio).0.43NH4++0.57NO2−+0.28HCO3−+0.06H+→0.44N2↑+0.11NO3−+0.028CH2O0.5N0.15+0.87H2O
To conduct the denitrification treatment using the ANAMMOX bacteria, it is preferable to set the molar proportion of nitrite-nitrogen to 1 mole of ammonium-nitrogen to be 0.5 to 2, especially 1 to 1.5, (that is, ammonium-nitrogen/nitrite-nitrogen=2/1 to 1/2, preferably 1/1 to 1/1.5). In the treatment of raw water containing ammonium-nitrogen, it is therefore preferable that a part of the raw water is nitrified by nitrification of nitrous acid type and the rest of the raw water containing the ammonium-nitrogen and the nitrified liquid containing the nitrite-nitrogen are mixed so as to prepare water to be treated or that a part of the ammonium-nitrogen in the raw water is converted to nitrous acid so as to prepare water to be treated.
Conventionally, control methods in the nitrification process for achieving nitrification of nitrous acid type have been studied where ammonium-nitrogen is oxidized as far as nitrite-nitrogen in order not to generate nitrate-nitrogen. For example, JP 2000-61494A describes a method of utilizing inhibition by ammonium-nitrogen in a tank. JP H4-122498A describes a method of controlling dissolved oxygen (DO) concentration. EP0826639A1 describes a method of utilizing difference in growth rate of microorganisms. In these methods, only ammonium oxidizing bacteria is activated by inhibiting the nitrite oxidizing bacteria, thereby accumulating high concentration of nitrite-nitrogen in a reaction tank.
In the reaction of oxidizing ammonium-nitrogen to nitrite-nitrogen, since ammonium-nitrogen which is alkaline is oxidized into nitrite-nitrogen which is acidic, pH in the nitrification tank tends to decrease. As the pH decreases to 6.5 or less, the activity of the ammonium oxidizing bacteria significantly deteriorates. In this case, the oxidizing rate of the ammonium-nitrogen should be lower than the oxidizing rate in case of neutral range. To inhibit this, it is required to add alkaline chemicals such as sodium hydroxide to control the pH.
In the method of utilizing the inhibition by ammonium-nitrogen among the aforementioned conventional methods, ammonium-nitrogen remains at high concentrations in the treated water. Therefore, further treatment is required to oxidize the remaining ammonium-nitrogen to nitrate-nitrogen as a post process. This loses advantage of generation of nitrite-nitrogen.
In the method of controlling the DO, the concentration of oxygen as a matrix is reduced so as to lower the treatment activity per sludge. Therefore, it is hard to treat high loading.
In the method of utilizing difference in growth rate of microorganisms, concentration of sludge is not high due to chemostat reactor. Therefore, it is hard to treat high loading.